What is the difference between an intermediate event and a basic event in a fault tree?

There is no meaningful difference; intermediate and basic events are essentially the same thing.

Basic $o{\displaystyle \underset{2}{\overset{\phantom{}}{}}}$ vents require research projects to understand properly, while intermediate events can be more easily understood.

A basic event cannot be further subdivided, but an intermediate event can be$.$

Every basic event must lead to an intermediate event before the top event.